It is well documented (U.S. Pat. Nos. 4,999,398 and 5,817,714) that emulsions, especially silicone emulsions, are opaque, cloudy, and tend to separate on standing. Microemulsions, in contrast, are desirable, because they are thermodynamically stable, and contain equilibrium microstructures that are smaller than typical emulsion droplets. Thus, microemulsion products are generally indefinitely stable, and they can be optically clear.
As used herein, the term emulsion or macroemulsion means a mixture of one immiscible liquid in another, in the form of droplets, with diameters approximately in the range of 100-1,000 nanometer (0.1-1.0 micron/1,000-10,000 angstrom .ANG.). In contrast, a microemulsion means a single or one phase transparent, thermodynamically stable, mixture of two or more immiscible liquids, and one or more surfactant(s) and co-surfactant(s).
In order to avoid confusion, it should be noted that the term microemulsion has been used loosely in the literature to describe any transparent composition containing water, oil and a surfactant, including compositions which are transparent by virtue of a very small structure size, and index of refraction matching.
However, it is almost always apparent from the details of the preparation given, which type of composition is in fact being made, considering the order of addition of the components, their polymerization, or when high energy mixing is involved.
Microemulsions, on the other hand, are generally always clear or transparent, because they contain structures smaller than the wavelength of visible light, typically of the order of magnitude of about 500 nanometer. Furthermore, a microemulsion, as that term is used herein, contains structures that are spontaneously self-assembled aggregates, consisting of oil and surfactant monolayers, or water and surfactant monolayers.
Although there are distinct domains present which are composed of water and oil, these types of systems can be properly described as being in the form of one phase, because the domains consist of molecular aggregates that spontaneously self-assemble.
A microemulsion may contain oil droplets dispersed in water (O/W), water droplets dispersed in oil (W/O), or it may be in the form of a bicontinuous structure or other structure. It can be recognized by several inherent characteristics which are (i) that it contains oil, water, and a surfactant; (ii) there is a high concentration of surfactant relative to oil; (iii) the system is optically clear; (iv) the phases do not separate by centrifugation; and (v) the system forms spontaneously.
For purposes of this invention, therefore, an emulsion is considered as containing structures having an average diameter of more than 100 nanometer (0.1 micron/1,000 angstrom .ANG.), whereas a microemulsion contains structures having an average diameter of less than 100 nanometer (0.1 micron/1,000 angstrom .ANG.), preferably less than 50 nanometer (0.05 micron/500 angstrom .ANG.), and most preferably less than 10 nanometer (0.01 micron/100 angstrom .ANG.). Microemulsion containing structures having an average diameter of as small as 5 nanometer (0.005 micron/50 angstrom .ANG.) is also contemplated herein.
Clarity or transparency is controlled to a great extent by the structure size of the dispersed phase. The scattering of light is dependent on the structure size. Therefore, clear or transparent compositions, according to this invention, are generally a single phase without droplets or structures, when viewed with the naked eye.
Furthermore, while emulsions are recognized as inherently unstable systems separating with time, microemulsions according to this invention, can be formed spontaneously, and are generally stable indefinitely.
It should be noted that true microemulsions of cyclic siloxanes in water are quite rare. Yet, classical methods of microemulsion formation have been known for many years in which a non-polar organic oil, water, a surfactant (S1), and a co-surfactant (S2), are mixed to form microemulsions, alleged to be thermodynamically stable. An adaptation of the classical method, coined the titration method, has even achieved some degree of notoriety in microemulsion circles, whereby organic oils, but not silicone oils, are added to a solution of a surfactant S1 and water to form two-phase mixtures. A co-surfactant (S2) is then slowly added with mixing, i.e., the titration, until the system becomes clear. It is said that microemulsions of particle size of 4 nanometer to about 20 nanometer can be formed in this fashion.
Advantage is taken of a titration method in this invention, in order to facilitate arriving at the production of silicone oil-in-water microemulsions, which can contain high molecular weight silicone polymers, with structures so small that they appear invisible to the naked eye, as a practical matter.